1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for obtaining a tomogram image of an object by using ultrasonic waves. The present invention is used mainly as a medical instrument for diagnosis.
2. Description of the Related Art
Recently, an ultrasonic imaging apparatus is widely used as a medical instrument for obtaining a tomogram image of an object, for example internal organs of a human body. Basically, the ultrasonic image apparatus has a plurality of electroacoustic transducers positioned in aligned relationship along the surface of the object. The electroacoustic transducers emit ultrasonic waves to the object and detect the ultrasonic waves reflected therefrom. The detected ultrasonic waves are added to each other, after a phasing process (i.e., which matches the phases of the detection signals) in accordance with a predetermined delay process for ultrasonic waves, to obtain the phased detection signal, i.e., a phase-matched, and thus strengthened, detection signal. After the above processes, it is possible to obtain an image signal on one scan line focused to one particular point on the scan line in the object. When electrically scanning the electroacoustic transducers, the focused point is straightly scanned (i.e., scanned linearly with parallel scan lines) so that it is possible to obtain the tomogram image of the object. Further, when controlling the time difference (also referred to below as the amount of delay), the focused point is curvedly scanned (i.e., as in a sector scan, with radial scan lines) so that it is possible to obtain the tomogram image of the object.
In this case, a propagation speed of the ultrasonic wave is slightly different within the object caused by the medium forming the object. That is, the propagation speed for the adipose tissue is different from that of the muscle tissue. Accordingly, the delay process is very important for adjusting the different propagation speeds of the ultrasonic waves to obtain a clear tomogram image of the object. That is, when the propagation speed is not uniform within the object, it is impossible to obtain the clear tomogram image of the object. Accordingly, it is necessary to provide means for preventing the deterioration of the tomogram image caused by nonuniformity of the propagation speed of the ultrasonic waves within the object.